Just by the stairs, you can access the remains of the Roman tower, also known as “Neptune Tower”. These circular, solid towers were built on a strong foundation of large carved stones taken from dismantled Roman monuments. Neptune Tower gets its name from a fragment of a funerary monument, still in its original spot within the tower’s foundations, depicting the god Neptune. It is the twin sister of Jupiter Tower, visible from the far end of rue du Marché au Beurre. You can also visit this tower by taking a short detour.